What Is Topsoil Used For?

Earth's natural topsoil layer is where minerals from deep inside the crust — pulverized into fine granules by natural processes — meet the organic matter left by eons of decomposing plant and animal life. It's the nutrient-rich substrate where plant life takes root, mycelial networks sprout mushrooms, and the microorganisms that make it all happen thrive.

Not surprisingly, topsoil is an important tool for landscaping and gardening, so it's available at garden centers everywhere and from wholesalers for about $55 per cubic yard. Besides providing nutrients, topsoil retains moisture, and when it washes away in rainy weather, it needs to be replenished. Generally, a 4- to 6-inch layer is needed for healthy lawns and gardens.

So, what is topsoil used for, what's the difference between topsoil and other types of garden soil — and what's the best method for actually placing it in your garden? We're here to answer all of your burning topsoil questions.

Go into a forest, pick a spot, clear away the leaves and other loose organic matter, and the first layer of soil you encounter is topsoil. This layer forms as leaves, wood, and other organic matter decompose and break down into small particles in a process that can take hundreds of years. The soil is dark and full of nutrients, and it's also loose, which makes soil erosion a problem when the topsoil layer is left exposed.

Now go to a riverbank or some other place where you can see a cross-section of the soil structure, and the difference between the layer of topsoil and the subsoil layer underneath it will be obvious. The topsoil layer is dark and sometimes black because of the organic material it contains, while the subsoil layer, which is a mixture of sand, silt and clay, is denser and lighter in color.

Topsoil varies not only in the amount of organic material it contains but also in its mineral composition. The combination, which is often characteristic of where the topsoil comes from, lends differences in the size of particles the soil contains, and they affect its texture and density. There are six main types:

Silt is composed of fine granules that compress easily, so silty soil is finely textured and holds water well. Silt particles are light and wash away easily, which makes soil erosion more of a problem, and because the particles compress so closely, silty soil drains poorly. However, silty topsoil holds nutrients well and is considered to be the most fertile type of soil.

Clay soil has a high ratio of minerals to organic matter, and it consists of very small particles. It can hold a lot of water, and when it gets wet, it is dense and slippery, but when it dries out, it becomes hard and brittle. Because of its density and water retention, clay topsoil is prone to compaction and often needs aerating. The best types of plants to grow in clay topsoil are trees and many hardy perennials.

Like clay, sandy topsoil has a high mineral content, but the particles are larger and have more space between them, giving the soil a granular feel. Sandy soil has a hard time retaining water and nutrients. However, because it drains well and heats up quickly in the sun, it provides a good growing medium for grass, although it needs more water and nutrients than other types of soil.

Chalky topsoil occurs in rocky areas and often has a layer of limestone or bedrock underneath it, which gives it its name. Chalky topsoil is loose, dries out quickly, and is highly alkaline, all of which make it one of the most difficult types of topsoil to work with. Plants grow poorly in chalk soil, and unless the soil is heavily amended, plants can develop chlorosis, a condition defined by yellowing leaves and stunted root growth.

On the other end of the pH scale, peat soil has a high concentration of organic matter that makes it acidic. Obtained in peat moss bogs, peat soil is loose and easy to work with. It heats up quickly in spring and holds water so well that soil amendments may be needed to loosen the soil structure and allow plants to breathe. Because of its acidity, peat soil slows the decomposition of organic matter and may starve the underlying soil of nutrients.

Widely considered the best soil for growing most plants, loam isn't a single type of soil but a mixture of sand, silt, and clay. The mixture helps neutralize the negative characteristics of each soil type. Loam is rich, dark, and loose. It holds moisture and nutrients better than other soil types, but it's susceptible to erosion.

Screened topsoil has been filtered through mesh to remove large particles and create a fine soil. This improves drainage and nutrient distribution and makes the product easier to handle. Unscreened topsoil, on the other hand, may include sticks, stones, and chunks of soil. It's a little less uniform but still a valuable garden amendment; however, it's not as good for lawns. Some landscapers refer to screened topsoil as lawn topsoil.

Although topsoil can be a combination of all the different types of soil in various proportions, it's usually fairly loamy, and gardeners and landscapers use it to amend garden beds and lawns. It contains the nutrients plants and seedlings need to survive, and it helps balance the soil pH and improve drainage.

Bagged topsoil has typically been blended to provide particular benefits, such as an abundance of chalky soil to cure acidity or peat to remedy alkaline soil. A blend with a high clay content can help the garden or lawn retain water, while one with more sand improves drainage. It's important to choose the blend you need based on the type and condition of the existing soil, so it's a good idea to conduct a soil test.

Landscapers also use topsoil to correct problems like soil depletion from erosion in existing lawns, and they will spread a layer of topsoil before planting a new lawn. For large areas, like lawns, topsoil is available to purchase in bulk by the cubic foot or cubic yard, but you can also use bagged topsoil, such as Scotts Turf Builder LawnSoil, for spot repairs where erosion or heavy traffic has caused damage.

Grass roots grow 4 to 6 inches deep, so if you're planting a new lawn, that's how deep your topsoil layer should be. To determine how much you need to order from the supplier, calculate the area of the lawn in square feet and then multiply that by 1/2 for a 6-inch layer and 1/3 for a 4-inch layer to get the result in cubic feet. Bulk topsoil is usually sold by the cubic yard, so divide that number by 27 to make the conversion to cubic yards.

When you're planting a flower or vegetable garden, a raised bed, or several container gardens on the back patio, there should be 8 to 12 inches of topsoil sitting on top of the garden soil. You can use the same formula to calculate the amount you need, although now you multiply the area of the garden (in square feet) by 2/3 to get the result in cubic feet. Topsoil is usually supplied in 1-cubic-foot bags, so the result tells you exactly how many bags you need.

When planting a new lawn, start by grading the existing soil with a 1 to 4 percent slope for good drainage and then spread the 4- to 6-inch topsoil layer over it using a rake. Sandy and clay topsoil benefits from a layer of organic mulch, such as peat moss or compost, which can be spread in a 1- to 2-inch layer over the topsoil before broadcasting the grass seed.

If you're creating a new plant bed or a container garden, mix in 2 inches of topsoil with the existing garden soil — being sure to use topsoil that is compatible with the existing soil — and then turn the soil with a trowel or shovel to mix it. This will create a transition zone between the existing soil and the topsoil. Afterward, spread the topsoil to a depth of at least 6 inches.

Yes, you can add topsoil to parts of an existing lawn or to the entire lawn. It's called topdressing, and it can cure such problems as depressions, dead or dying grass, and excessive thatch. Before you proceed, be sure to test the soil and topsoil first to make sure they are compatible and if they are drastically different, mix peat, clay, or sand into the topsoil as needed. (If you need assistance with this, contact your local gardening center.)

If you're spot-filling a low area:

1. Pour the soil into the depression to bring the soil level slightly higher than the rest of the lawn to allow for settling.
2. If you're trying to revitalize dying grass or cure thatch, remove the thatch with a rake, aerate the area, spread a layer of topsoil 1/8 to 1/4 inch thick, and work it into the soil.
3. Follow up by spreading grass seed on the area.

You can also apply a thin, 1/4-inch layer of topsoil over the entire lawn to level it, control thatch, or improve the existing soil. Remove thatch and aerate as needed and spread the soil with a broadcast spreader.

Fill dirt is different than topsoil. Fill dirt has a high mineral content and usually contains rocks, sand, and clay. It has very little organic matter, so it doesn't support plant growth very well if at all. Topsoil, on the other hand, is rich in organic matter. Whereas fill dirt is often compacted and drains poorly, topsoil is loose and well-draining. Fill dirt provides a solid base for a garden bed or container garden, but topsoil is the growing medium.

Unlike topsoil, potting soil isn't soil at all. A typical potting mix is a mixture of organic or plant-based materials, such as rice hulls, composted tree bark, bat guano, or earthworm castings, combined with inorganic materials, like perlite, vermiculite, or sand, to make a very lightweight growing medium for a healthy plant. Fertilizer is often added to potting mix along with pH balancers, microbes, and moisture-retaining additives.

Because it's lighter, looser, and drains better than topsoil, potting mix provides a better ecosystem for growing healthy plants in pots and containers. Topsoil, on the other hand, is a better choice for in-ground gardens and lawns.